Readability. Possibly the single most important proposal feature. I'll take a clear proposal on decent science over a convoluted proposal on groundbreaking work, 7 days a week and twice on Sunday.
Especially for NSF preproposals, if you are not writing for a tired reviewer who is not in your field, you are doing it wrong. Remember, there are no ad hocs on these proposals, so unless you are working on an obvious model in a big field, chances are many or most of the reviewers won't have a background in what you are proposing. This differs, to a degree, from full proposals. However, readability is still critical.
Before you submit, have someone outside your field read your proposal, asking them specifically to comment on their understanding of it. I can't tell you how many proposals I have read with 5 line sentences that take the reader on a tortious voyage. No matter how eloquent you think you are being with these long-winded soliloquies, I beg you to reconsider.